As a homeowner, you obviously have an excellent head start compared to a heating, venting and air-conditioning professional on knowing which rooms are warmer than others since you live in your house. Because your system uses one forced-air heating source, when you adjust an air vent in one room, it can change the airflow throughout the house. Adjusting individual air registers to create the appropriate airflow in each room may take several days of experimentation before you get it just right.
Visit each room and if necessary open all parts of the forced-air heating system -- floor, wall or ceiling registers or diffusers, typically measuring around 4 by 10 inches -- and the larger, square returns, which may measure around 12 inches square, in the floor or lower wall. The returns don't have built-in dampers but may have been blocked off with a piece of cardboard or a magnetic cover; if so, remove this. These returns pick up the air in the home and send it back for heating.
Move away any furniture in front of the returns or drapes that block them so they can function properly.
Replace the furnace air filter in the furnace filter cabinet, found in the largest air return next to the plenum, if you have not done so in the last 90 days to ensure the filter is clean. If you have a permanent filter, clean it by vacuuming and reinstall.
Visit each room to evaluate which receives the most heat and which ones receive the least. If you've lived in the house a while, you probably already know which rooms receive more heat than others; typically higher floors and those with southern exposure have more heat gain. If you are new to the home, you must run the heating system to check this. You can also use an indoor thermometer positioned in each room and check it after the furnace has been on for 30 minutes to determine the rooms that receive more heated airflow.
Adjust the air vent by partially closing the register in rooms with too much heat and airflow. Leave the air registers open for larger rooms. Rooms closer to the air source might require complete closure, as the heated air still moves through the louvers in the vent because they are loose, even when they are fully closed.
Re-check each room with the air registers adjusted for the room's desired temperature. Readjust as necessary until the comfort level that you desire is achieved.